1. Field
The present invention relates generally to the field of communications and more specifically to providing specific gain control characteristics in Radio Frequency (RF) driver amplifiers (DAs).
2. Background
In a wireless communications system, a user having a terminal (e.g., a cellular phone) communicates with another user via transmissions on the downlink (forward link) and the uplink (reverse link) through one or more base stations. Downlink refers to transmission from the base station to the terminal, while uplink refers to transmission from the terminal to the base station.
Cellular telecommunications systems, such as Code Division Multiple access (CDMA) communications systems, are often characterized by a plurality of mobile stations, or terminals (e.g. cellular telephones, mobile units, wireless telephones, or mobile phones) in communication with one or more Base Station Transceiver Subsystems (BTSs). Signals transmitted by the mobile stations are received by a BTS and often relayed to a Mobile Switching Center (MSC) having a Base Station Controller (BSC). Alternately, mobile station transmissions may be received by a BTS and relayed to a Public Data Serving Node (PDSN) through a BSC. The MSC and the PDSN, in turn, route the signal to a Public Switched Telephone Network (PSTN), a data network, or to another terminal. Similarly, a signal may be transmitted from the PSTN or data 00network to a terminal via a base station or BTS and an MSC, or via a BTS, a BSC, and a PDSN.
The output stage of a wireless communication device, or terminal, employed in connection with the foregoing wireless communication system typically includes amplifiers that strengthen the radio frequency (RF) transmission in the foregoing system. For example, the wireless communication device may be a CDMA wireless communication device or terminal that employs one or more RF amplifiers to provide an adequate radar frequency signal transmission.
In a direct conversion transmitter architecture, controlling RF driver amplifier gain is generally desirable for a variety of reasons. For example, CDMA standards require a transmitter having approximately 90 dB of gain control range. Typical high volume, manufacturable single stage Variable Gain Amplifier (VGA) circuitry can only attain in the range of approximately 60 dB of gain control range. As the VGA is typically located at the output of the direct upconverter in the design shown, it can be difficult if not impossible to increase the gain range in the presence of previous VGA circuitry.
Linear in decibel gain control characteristics provide certain advantages in CDMA applications. Power control requirements in CDMA, for example, require tight control over the output power of the terminal. Phone output power is preferably calibrated and repeatable against the received power control voltage. Total average power consumption is preferably kept to a minimum, and power consumption can be reduced in the presence of a variable gain at the RF driver amplifier.
When implementing a gain control function in a driver amplifier, adequate linearity and noise performance must be available when delivering a significant level of output power, such as in the range of approximately 10 dBm. Linear in decibel gain control is particularly difficult due to package parasitics and bondwire inductances in the integrated circuit design of the driver amplifier.
Previous RF systems seeking enhanced gain control over the 90 dB range have employed different designs with mixed results. For example, certain designs use multiple VGAs, as a single VGA only generally provides 40 to 60 dB of gain control. A dual VGA setup can increase the gain control range, but this design is difficult to operate at a single frequency in the desired operational frequency range, and is difficult to tune, bias, and calibrate properly. A dual VGA system can be used for a dual conversion (superheterodyne) architecture, as each VGA can operate at a different frequency in such a design. Such a design can be undesirable because of current drain, additional required circuitry and more complex circuitry, requires more area, and is more expensive in an IC circuit design. Generally speaking, any design employing multiple VGA circuits or designs having the effect of multiple VGA circuits are undesirable in direct conversion systems in particular.
Previous designs have also employed transmit integrated circuits within the terminal to use a variable bias current in the drive amplifier. Variation in the output current has been observed to be on the order of four to one over the full gain control range. While this can reduce current drain at low output power levels, the gain is not varied in an appreciable manner in this implementation.
It would therefore be advantageous to provide a RF driver amplifier linear in decibel variable gain design for direct conversion transmitter applications, particularly variable linearly over the range of zero to 90 dB that overcomes the drawbacks of previous designs.